Documentation improvements

meson build/ -Dgtk_doc=true
ninja -C build/ libmanette-doc

leads to

Building documentation for libmanette
Traceback (most recent call last):
  File "/usr/bin/meson", line 11, in <module>
    load_entry_point('meson==0.55.1', 'console_scripts', 'meson')()
  File "/usr/lib/python3.8/site-packages/mesonbuild/mesonmain.py", line 228, in main
    return run(sys.argv[1:], launcher)
  File "/usr/lib/python3.8/site-packages/mesonbuild/mesonmain.py", line 217, in run
    return run_script_command(args[1], args[2:])
  File "/usr/lib/python3.8/site-packages/mesonbuild/mesonmain.py", line 165, in run_script_command
    return module.run(script_args)
  File "/usr/lib/python3.8/site-packages/mesonbuild/scripts/gtkdochelper.py", line 248, in run
    build_gtkdoc(
  File "/usr/lib/python3.8/site-packages/mesonbuild/scripts/gtkdochelper.py", line 125, in build_gtkdoc
    shutil.copyfile(f, os.path.join(abs_out, os.path.basename(f)))
  File "/usr/lib64/python3.8/shutil.py", line 261, in copyfile
    with open(src, 'rb') as fsrc, open(dst, 'wb') as fdst:
IsADirectoryError: [Errno 21] Is a directory: '/var/home/moi/Projets/gnome/libmanette/doc/'
ninja: build stopped: subcommand failed.

but I basically copied all this thing from libdazzle/libhandy (adapted it) so… I don't know.

Needs the libmanette-docs.xml thing as well and also copy the doc/xml directory from libhandy.

Fixes #20 (closed)